مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2871
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that lbn 'Abbas (رضي الله عنه) said: The Messenger of Allah (ﷺ) used to teach us the tashahhud as he would teach us a soorah from the Qur'an.
Musnad Ahmad 2892lts isnad is saheeh, Muslim (403)]